As will be understood by those familiar with the investment casting art, the general procedure for making solid investment molds involves attaching patterns having configurations of the desired metal castings to a runner system to form a set-up or “tree”. The patterns and runner system are made of wax, plastic or other expendable material. To form the mold, the set-up or tree consisting of the pattern or patterns attached to the runner system are placed into a flask which is filled with a refractory investment slurry that is allowed to harden in the flask around the tree or set-up to form the mold. After the investment slurry is hardened, the patterns are melted out of the mold by heating in an oven, furnace or autoclave. The mold is then fired to an elevated temperature to remove water and burn off any residual pattern material in the casting cavities.
Conventional investment formulations used for non-ferrous molds are comprised of a binder and a refractory made up of a blend of fine and course particles. A typical refractory usually is wholly or at least in part silica, such as quartz, cristabolite or tridymite. Other refractories such as calcined mullite and pyrophyllite also can be used as part of the refractory.
Gypsum powder (calcium sulfate hemihydrate) is almost universally used as a binder for molds intended for casting gold, silver and other metals and alloys having relatively low melting points. Casting of high melting point metals and alloys, such as platinum and platinum alloys, precludes the use of gypsum bonded investments. Instead, phosphate bonded investments have been and continue to be used to some extent for casting such metals. One type of phosphate investment generally comprises a refractory powder such as silica and a binder formed by reacting phosphate salts or phosphoric acid with a reactive oxide. Typically used phosphate investments generally utilize monoammonium phosphate and magnesium oxide with the latter being present in greater than the stoichiometric quantity required to react with the phosphate salt. Sometimes monomagnesium or monocalcium phosphate have been included to modify the performance of the investment. Illustrative examples of phosphate investments are disclosed in U.S. Pat. Nos. 2,072,212; 2,152,152; 2,209,035; 2,680,890 and 2,928,749.
Phosphate investments of the type described above, while known and available for many years, have not been completely satisfactory for platinum jewelry, and efforts have been made by others to provide improved investments for this application. A recent development that has become commercially available consists essentially of silica ground to a relatively fine particle size which is mixed with dilute phosphoric acid. Phosphoric acid investment does not set up in a cementitious manner like gypsum bonded investments. Since the investment does not set like a cement, the slurry is poured around the pattern cluster which is mounted on an absorbent surface within a metal flask. The flask is provided with a removable paper extension on its top so that the slurry can be higher than the top of the flask. The mold is then dried further, dewaxed and heated to prepare it for casting. The use of the absorbent surface under the flask permits the mold to dry from both the top and bottom surfaces.
Although the phosphoric acid type investments are capable of producing good castings with smooth surfaces, there are a number of objections to the use of phosphoric acid. It is a hazardous material and requires Hazmat packaging which increases shipping costs. In order to minimize this cost, the acid is usually provided at a high concentration, such as 50 to 85 percent phosphoric acid, which the customer must store, dilute and measure out as needed according to the supplier's specifications. The use of any additive which must be measured out and added to the mixing water, whether solid or liquid, and whether hazardous or not, requires additional in-plant operations and is a potential for mistakes. The use of a hazardous chemical has the additional disadvantage of adding to in-plant safety concerns. All of these disadvantages are particularly burdensome to users who purchase and use investment materials in small quantities.